Oldman in The Professional is tasty as well...
the witch form The Wizard of OZ is worth mentioning

Anti-hero/villain from "Man Bites Dog", get the unrated version.

I love that movie! Kingsley is simply menacing in the role. A couple more memorable villians I would add:

Rodney McDowell, "A Clockwork Orange"
Christopher Walken, "The King of New York"
Nicholas Cage, "Kiss of Death"
Wes Studi (who played Magua), "Last of the Mohicans"
Laurence Olivier, "Marathon Man"
The prison guard in "Midnight Express"

Not trying to be persnickety, Videodrome, but Malcolm McDowell is in A Clockwork Orange. Funny, when I put this question to my wife, he was the first guy she came up with. I think the fact that the film is over-the-top satiric makes me kinda not register the "droogs" as evil—though they are, of course.

tru blu: My bad. I got the wrong McDowell!

The scene that really stands out in terms of cruelty was the home invasion: McDowell kicking the guy on the floor while belting out verses from "Singing in the Rain," was really sick and twisted.

Although I don't recall ever seeing the bad guy in SE7EN he was one evil SOB. They only referred to him as John Doe if I recall correctly.

How about Christopher Lee as Saruman in Lord of the Rings?

John Doe was shown quite clearly in the film, played by Kevin Spacey. The interesting thing about Doe is that he thinks he's doing a good thing by creating this amazing piece of performance art that point out our sinful side. In fact, he even talks about how he's doing society a favour by offing his 'victims'.

Those are my favourite bad guys: the ones that don't know they're bad guys. One of the two villians in 'Mission Impossible: 3' carefully explains to the Tom Cruise character how America will benefit by middle-eastern terrorists getting a super powerful bio-weapon (the film's McGuffin). In his mind it makes sense, and will eventually end up being good for everyone.

You find these types of bad guys in fiction (Magneto from 'X-Men') and reality (Hitler, Bin Laden). They are absolutely convinced they're making the world a better place. Actors who play bad-guy roles keep an old saying in mind when trying to find motivation for their character: "the villian is the hero of his own story".

These do make some of the best "villians."

An extreme example of this comes to mind. There's a long-running science fiction series by David Weber that includes a war between two nations, one of which goes through about three changes in leadership during the course of hostilities. By the time the last leadership group assumes control, the purpose behind the war and the reasons that it continues have been lost - both sides are being led politically and militarily by well-intentioned people, with only a few destroyed diplomatic documents and one totally random death keeping the two sides from seeing eye-to-eye.

Now see, sdurani, I'd say that the cats you describe definitely know they're bad guys, they're just delusionally sociopathic. Another word for that is "crazy." As for Hitler and bin-Laden, I think you could also say that a pathological need for vengeance played/plays a part in their motives.

gonk, Weber's scenario sounds an awful lot like current conflicts in any number of parts of the world. If you hadn't said it was sci-fi, I might not have guessed it was such.

One more interesting bad guy: Keyser Söze, Kevin Spacey in The Usual Suspects.